1. Field of the Invention
This invention relates to a transparent co-extruded acrylic sheet having use as mar resistant, non-glare picture frame covers and for other purposes.
2. Prior Art
The most common materials used in picture frame cover applications are glass and acrylic. Within each of these classes of materials there are standard frame materials that are glossy and there are specialty frame materials that are non-glare or less glossy.
The standard glass covers include standard plate glass or window glass and these materials are quite commonly used in these applications. The standard acrylic picture frame covers are also commonly used and examples of these include Plexiglas® MC (an extruded and melt calendered acrylic sheet comprised of a MMA/EA copolymer having a melt flow rate of about 2 g/10 minutes when measured in conformance with ASTM D 1238, Condition 230° C./3.8 kg, procedure A) and Plexiglas® UF—5(an extruded and melt calendered acrylic sheet comprised of a MMA/EA copolymer having a melt flow index of about 2 g/10 minutes when measured in conformance with ASTM D 1238, Condition 230° C./3.8 kg, procedure A; wherein a substantially higher amount of an ultraviolet filtering agent is added) sheet products.
However, a deficiency of each of these products is that they are glossy. Hence, it is common to see reflections (particularly from overhead lighting) when viewing the images protected by these types of picture frame covers. To overcome this deficiency, both glass and acrylic manufacturers typically impart a very fine pattern onto the outside surface of the picture frame cover. The patterned surface reduces the specular reflections or gloss by scattering the light. This reduces the glare on the protective cover.
Even though the concept of imparting a pattern onto the surface to achieve a non-glare finish is shared by both glass and acrylic manufacturers, the approach to achieve it is very different. Typically, glass manufacturers etch the surface of the glass with an aggressive acid in order to achieve the desired texture. Whereas, the state-of-the-art for producing plastic non-glare picture frame covers is via the sheet extrusion process where an embossing roll is used to impart a non-glare pattern into the molten polymer extrudate as it is being polished and cooled into sheet.
An example of this type of product is Plexiglas® Non-Glare sheet (an extruded and melt calendered acrylic sheet comprised of a MMA/EA copolymer having a melt flow rate of about 2 g/10 minutes when measured in conformance with ASTM D 1238, Condition 230° C./3.8 kg, procedure A); wherein an embossing roll is used to mechanically impart a pattern onto the surface of the sheet on-line during the polishing steps.
However, this embossing technology in the sheet extrusion process has several shortcomings:
1. The extrusion line needs to be shut down in order to change one of the polishing rolls from a typically used highly polished chrome roll used to produce standard products such as Plexiglas® MC sheet to a patterned roll typically used for a specialty product such as Plexiglas® Non-Glare sheet.
2. The degree of non-glare or gloss of the resultant Plexiglas® Non-Glare sheet changes dramatically for each sheet thickness produced. Typically with this embossing technology, a much higher gloss results on the thicker gauge sheet. This is because the thicker sheet retains the heat necessary to extrude or melt calendar the material for a much longer period of time. Consequently, after the sheet has been embossed with the pattern, the thicker gauge sheet tends to relax more and deviate more from the original embossed pattern.
The mar resistance properties of extruded acrylic sheet is a deficiency especially when compared to glass. Abrasion resistant coatings may be used to help improve this property but this added processing step is quite expensive. An example of this type of product is Acrylite® AR (a melt processed acrylic sheet comprised of MMA/MA composition which has an abrasion resistant hardcoat applied onto the surfaces) which is commercially available from Cyro Industries.
The present invention enables an acrylic sheet to be economically produced using a co-extrusion process wherein:
1. The line does not need to be shut down to change from a highly polished sheet product to a specialty non-glare sheet product.
2. The gloss properties of the resultant sheet at various thicknesses are much more consistent when compared with sheet produced using the embossing technology.
3. The mar resistance properties of the resultant sheet are improved compared to typically extruded acrylic sheet product all in one processing step.